Fish can be fooled—just like us

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People might have more in common with fish than previously thought, a new visual illusion study indicates.

UQ School of Biological Sciences and Queensland Brain Institute researchers, who published their findings in Scientific Reports have found that triggerfish, a common Great Barrier Reef species, are fooled by visual illusions in the same way humans are.

One of the study authors, Dr Karen Cheney, said that she found illusions a fascinating area of research.

“They demonstrate situations when our brains fail to perceive the true properties of what we are looking at, in terms of colour, brightness, size, shape and movement.”

“It is not entirely clear why we see illusions,” she added, “but there are a few different ideas. Overall, they occur due to processes on the retina or in neural pathways closer to the brain.”

To ascertain whether fish perceive complex illusions similar to us, Dr Cheney and her colleagues subjected them to what is known as the “lightness cube illusion test”.

In this test, which still baffles observers and is not fully explained by science, an orange square on the top of a cube is perceived to be different from a brown square on the cube’s side, even though they are actually the same in colour and brightness.

“We think the illusion occurs because the ‘brown’ is perceived to be in shadow and therefore our brain encodes it as darker than the ‘orange’ square, which is perceived to be out of the shadow,” Dr Cheney said.

Wondering if the illusion also occurred for fish, they conducted the same illusion experiment, after first training the fish to approach and peck at either an orange or brown coloured square to receive a food reward.

“The fish preferentially chose the square that was most similar to their trained colour,” said Dr Cheney, “indicating they also ‘think’ the target is lighter-coloured and that they perceive the two squares to be different.”

Dr Cheney said the study was further confirmation that non-human species—guppies and butterflies are some others—perceive illusive objects in apparently the same way as humans.

In addition, as triggerfish were the subject of this study, it may be that scientists need to think differently about how inhabitants of brightly coloured coral reef environments perceive their surroundings.

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